In motor graders, the rear wheels are commonly driven directly by the engine through a transmission and differential gearing. Further, it is common to supplement the main drive by means of a hydrostatic front wheel drive system. More specifically, the supplemental drive typically includes a fluid pump driven by the engine for providing pressurized fluid to fluid motors. The motors in turn drive the front wheels thereby supplementing the main rear wheel drive. In the past, supplemental hydrostatic drives have been developed which automatically shifted between two or more torque levels in response to the transmission ratio and/or hydraulic system pressure. These systems were continuously powered to provide supplementary hydrostatic drive for the main drive but had no provision for operation only on demand when the main drive loses traction. However, since the supplemental hydrostatic drive is notably less efficient than the main direct drive, it is desirable to reduce unnecessary utilization of the hydrostatic drive.
Recent attempts to overcome the problems associated with these systems have included electronic controls for varying the speed of the front wheels in response to the speed of the rear wheels. More specifically, these systems commonly utilize speed sensors to monitor front and rear wheel speeds and a closed loop electronic control for varying pump displacement to maintain a preselected speed relationship between the front and rear wheels.
In some motor graders manufactured by the assignee hereof, the vehicle operator is able to manually control the torque produced by the supplemental drive. These vehicles include an open loop control wherein the pump and motor displacement in the supplemental drive are adjusted in response to a manually operable control lever. However, these systems do not provide maximum efficiency since control of the hydrostatic drive is at the operator's discretion. Therefore, it is desirable to provide a system in which operation of the hydrostatic system is automatically controlled to maintain maximum operating efficiency.
One way in which automatic control of the hydrostatic drive can be achieved is through the use of a closed loop feedback system.
One such system is disclosed in U.S. Pat. No. 4,186,816 which issued to Pfundstein on Feb. 5, 1980, hereinafter referred to as '816. The '816 patent discloses a closed loop electronic speed feedback system for automatically controlling the supplemental drive system of a motor grader. More specifically, closed loop feedback electronics control a servo actuated pump that is connected by a hydraulic system to a pair of front wheel hydrostatic drive motors. The supplemental drive for the motor grader has three modes of operation. The first mode is the "off" mode in which the front drive wheels are free running and unpowered. The second mode is the "normal" mode where the control system allows a predetermined amount speed differential between the main and supplemental drive wheels before the supplemental hydrostatically driven wheels begin to supplement the main drive wheels. The third mode is an "overspeed" mode where the control system provides a predetermined percentage of overspeed of the auxiliary drive wheels to provide a continuous, positive supplementary drive.
As mentioned previously, hydrostatic drives are much less efficient than direct drives and, therefore, it is desirable to utilize the supplemental drive in the most efficient manner possible. The present invention is directed to that end.